The use of hydraulic cylinders to control the movement and ultimate position of components is well known. Typically such cylinders are linear, utilizing a cylinder with a piston dividing the cylinder into a pair of chambers and a piston rod attached to the piston and extending out of one or both ends of the cylinder. A control valve will then control the admission of fluid into one or other of the chambers, causing the piston to move relative to the cylinder and adjust the length of the hydraulic actuator.
In its simplest form, the control of the hydraulic cylinder is performed manually through a simple control valve. More sophisticated controls may control the movement of the cylinder in response to fluctuations in the supply pressure, flow rate of fluid or a combination thereof. Attempts have also been made to control the movement of the cylinder by monitoring the position of the movable component and providing a feedback signal to a control valve that indicates the difference between the desired location and the actual location of the component. Such arrangements rely upon the accurate measurement of the position of the piston relative to the cylinder but the measuring device must be sufficiently robust to withstand the normal harsh environment in which the actuators may be used and at the same time must maintain the structural integrity of the actuator.
U.S. Pat. No. 3,956,973 shows a hydraulic cylinder in which a series of magnetic discontinuities are provided by a square thread form cut in the wall of a cylinder rod. The discontinuities are detected by a transducer that detects the change in magnetic intensity as the thread passes the transducer. The transducers create pulsed signals which are used to provide data relating to position, velocity, and acceleration of the piston rod. The signal produced by the square thread form only identifies whether the top or base of the thread is adjacent the transducer and therefore does not give a resolution greater than the pitch of the thread. To enhance the frequency of the signal, an array of transducers is used that are distributed about a portion of the circumference of the piston rod. Whilst the frequency of the signal is increased, each transducer is only able to detect edges of the thread form and therefore the resolution is dependent on the number of transducers. Moreover, because the signal is either xe2x80x9chighxe2x80x9d or xe2x80x9clowxe2x80x9d there is an ambiguity upon reversal of the direction of movement of the rod that leads to complexity in the signal processing.
It is therefore an object of the present invention to provide an actuator in which the above needs are addressed.
In general terms, the present invention provides an actuator having a cylinder and a piston assembly moveable within the cylinder. The piston assembly projects from the cylinder and has a surface that presents to a predetermined location on the cylinder a periodically varying discernible characteristic as the surface moves relative to the cylinder. A sensor assembly monitors the discernible characteristic and measure changes therein to provide an indication of movement of the piston assembly relative to the cylinder. The varying discernible characteristic is provided by an interface on the piston assembly exhibiting a change in the discernible characteristic. The interface is inclined to the direction of travel of the rod whereby the distance between the interface and the transducer varies progressively and continuously as the piston assembly moves relative to said cylinder.
Preferably the surface of the cylinder is smooth to permit passage past a seal assembly.
In one embodiment it is preferred that the discernible characteristic is magnetic permeability and varying permeability is provided by a screw thread having convergent flanks formed in the outer surface and filled with a material of a different magnetic characteristic to present a continuous cylindrical surface to the piston assembly.
In a further embodiment, the magnetic to non-magnetic interface is provided by a tapered piston rod having a non-magnetic coating of varying thickness to provide a cylindrical outer surface.
As a further preference, a plurality of magnetic transducers are circumferentially spaced about the piston rod to enhance resolution of the movement of the piston assembly relative to the cylinder.